swift234

Reflection: Single-payload enum layout

Attempt to lay out single-payload enums, using knowledge of extra
inhabitants where possible.

- The extra inhabitants of an aggregate are the extra inhabitants of
  the first field. If the first field is empty, there are no extra
  inhabitants, and subsequent fields do not affect anything.

- Function pointers and metatypes have different extra inhabitants
  than Builtin.RawPointer, so have IRGen emit distinct builtin type
  descriptors for those.

- Opaque existentials do not have extra inhabitants.

- Weak references do not have extra inhabitants.

Also, fix IRGen to emit more accurate enum reflection metadata in
these two cases:

- We now record whether enum cases are indirect or not. An indirect
  case is the same as a payload case with Builtin.NativeObject.

- We now record whether a case is empty or not using the same logic
  as the rest of IRGen. Previously, we would incorrectly emit a
  payload type for a case with a payload that is an empty struct,
  for example.

At this point we don't have a way to get the currently inhabited
enum case from a value. However, this is still an improvement because
we can still reflect other fields of aggregates containing enums,
instead of just giving up.

Finally make some methods on TypeCoverter private, and use 'friend'
to allow them to be accessed from other internal classes, making the
public API simpler.